Extraterrestrial Life on Tidally Locked Planets

Can small, red dwarf stars (M class) really support extraterrestrial habitability? Red dwarf stars are much cooler (~10% brightness) and smaller (10 to 50%) than our Sun. This pushes the habitable zone for planets much closer to the parent star of the system which, in turn, causes planets to become tidally locked (one hemisphere always facing the star like the Moon is to Earth). Also, being closer the planets are more exposed to infrared and flaring radiation. Yet, they have incredible lifespans in trillions of years (vs. the Sun’s 10B year lifespan) and comprise about 75% of all stars in the galaxy.

Gale & Wandell (2017) argue that perhaps tidally locked planets orbiting red dwarfs may develop life in zones of “continuous illumination with a considerable component of photosynthetic 400-700 nm radiation. This is analogous to the lush summer growth of vegetation on Earth, at very high latitude.” You can read the paper here: The Potential of Planets Orbiting Red Dwarf Stars to Support Oxygenic Photosynthesis and Complex Life

With the large number of red dwarfs, maybe we’ll discover biotic markers in future missions!

0

Detected Habitable Planets?

Yesterday I mentioned Barnard's Star having four exoplanets but none of them are in the estimated habitable zone defined for a red M-dwarf parent star. So, where are we currently at for detection of potentially habitable planets? The current estimates are approximately 70 out of 5000+ detected exoplanets that are more likely to have a rocky composition and support surface liquid water. These would have varying parent stars that would typically be like our own Sun (G-dwarf) or cooler (K or M dwarfs)

0

What is the Closest Earth-like Exoplanet?

Artist View of M dwarf Exoplanet Sun

Recently I was asked this question by a friend which made me stop and ponder it for a minute. Of course, my mind jumped to my older posts on what an 'Earth-like' planet is along with the definition of a "habitable" planet (the key differentiator being Earth-like doesn't necessarily mean habitable or terrestrial).

This isn't an easy question to answer since we don't fully understand what habitable or terrestrial might look like for different exoplanet stellar systems. For example, if we're going to limit the question to exoplanets around F, G, K and M dwarf stars due to stellar lifespan (for example, bright A5V stars only have a lifespan of about 750 million years), then one current estimate is there are 65 billion terrestrial planets around these 4 classes of stars in the Milky Way galaxy. However, since cool, low mass M dwarf stars comprise 76% of the total, terrestrial exoplanets are going to be in tight, short orbits, most likely tidally locked with low UV stellar radiation. That doesn't rule out Earth-like habitable exoplanets, but that's a large unknown at this point (something we'll talk about in a future post). 

So, to get back to the question, let's pick F, G and K main sequence stars first. Using that criteria, 82 Eridani in Eridanus (visible from the southern hemisphere) is going to be the best answer for those stellar candidates. At about 20 light years away, the two planets, 82 Eridani e and f orbit a G6V star (very similar to the Sun) with a periods of 147 and 647 days potentially within the star's habitable zone. As of this writing, both fall on the edges of that habitability zone with e on in the inner edge and f on the outer edge. Also, the detected exoplanets masses are at least 4x that of Earth's. Given that, this is probably the best case for now. 

In the northern hemisphere, if you look in the constellation of Cassiopeia, the orange star Gliese 892 (stellar type K3V - cooler than the Sun) may be the closest at 21 light years. However, the main problem is the candidate exoplanet's mass at over 10x Earth's. Also, that particular exoplanet hasn't been confirmed.

Honestly, there's not much beyond this since we're dealing with more distant, brighter stars (see my mention of the Malmquist bias in my previous post). If we go back to lower mass, cooler M dwarf stars then my favorite pick is Gliese 667 in the constellation of Scorpius at 23 light years away. It is a triple star system with the third star, Gliese 667 C, having detected exoplanets. Planet e with an orbit of about 62 days and mass of about 3x that of Earth's is within the star's habitable zone. There are many other M dwarf star systems with detected exoplanets. If you're interested, you can browse the Habitable Worlds Catalog or NASA's Exoplanet Catalog to see what potential exoplanets could be candidates.

Given how difficult this question is, there just isn't a good answer yet. There are still so many assumptions along with unanswered questions as to just what extraterrestrial life is, how it may evolve and what types of stellar systems would support it. All we can do is continue to gather more data and assume life on Earth as the initial standard to measure against.

0

Webb on the Forefront of Earth-like Exoplanet Discoveries

After analyzing data from NASA's Transiting Exoplanet Survey Satellite (TESS), the research team led by Kevin Stevenson and Jacob Lustig-Yaeger from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland chose to observe the exoplanet LHS 475 b using the NASA/ESA/CSA James Webb Space Telescope. With only two transit observations, Webb's Near-Infrared Spectrograph (NIRSpec) captured the planet and confirmed its existence.

LHS 475 b is 41 light years away with a radius nearly equal to that of Earth and an orbital period of two days around an M-type (M3.5V) star in the southern hemisphere constellation of Octans. While the observation couldn't fully identify what atmosphere is present on the planet, it isn't a thick methane-dominated atmosphere. But, there is a possible carbon dioxide atmosphere, which is contrary to expectations due to its compact nature. Further observations with more precise measurements are being planned for this summer in order to distinguish between a pure carbon dioxide atmosphere and no atmosphere at all.

Clearly we're on the cusp of some significant exoplanet discoveries!

You can read more here 

0

24 Top Exoplanets to Live On - Is this Science?

Courtesy NASA

Sometimes I really like articles that nudge the scientific envelope a bit but many times I really don't like how the mainstream media reports it. You can see here how Gizmodo't title "These 24 Planets Might Be Better Places to Live Than Earth, Astrobiologists Say" might be a bit sensational - especially considering some of the jabs in the article.

That said, it's good news that some criteria for habitability is being pushed in the scientific journals. I will post more on this in the future.

0

Biosignatures on Venus

Possible NASA Havoc Mission to Venus

Recently, there was some very exciting news about possible biosignatures in an unexpected place within our Solar System. Amazingly, it was announced this month by Greaves et al. 2020 that Phosphine gas was detected in the cloud decks of Venus. What is so striking about this is that the levels of Phosphine measured do not match any known abiotic terrestrial mechanism, or, as the authors stated - "The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery."

 

Phosphine (PH3), a toxic gas, is commonly created by organic life typically deep in bogs and swamps (see Devai & Delaune 1995) as well as by microbial metabolism (see Glindemann et al. 2005). Though it can be created by other abiotic means, the levels were significantly lower than what was calculated from sunlight, meteorites, lightning or emissions from volcanism on Venus' surface. Even degassing from Venus' mantle rocks is about 10 orders of magnitude too low to produce PH3.  

It should be noted that the announcement is by no means a confirmation of a biosignature. The team clearly calls this out - Venus' surface composition, temperature and atmospheric interaction stills needs additional data. "This could be unknown photochemistry or geochemistry, or possibly life. Information is lacking—as an example, the photochemistry of Venusian cloud droplets is almost completely unknown. Hence a possible droplet-phase photochemical source for PH3 must be considered (even though PH3 is oxidized by sulfuric acid). Questions of why hypothetical organisms on Venus might make PH3 are also highly speculative". Interestingly, phosphine is also found in Jupiter's atmosphere thought to be caused by abiotic turbulent atmospheric mixing.

This is quite a remarkable discovery considering Venus' proximity and number of observations from both Earth and space observatories and probes which should spur a significant amount of funding for future Venus missions.

0

Mission to Europa

This is great news! The Europa Clipper is set to launch in 2025.

0

Nematodes Awaken after 40000 Years

In an amazing article by Shatilovich et al. (2018) multicellular organisms (in this case nematodes or small roundworms) were thawed back to life from Siberian permafrost demonstrating the amazing survivability of life in extreme conditions. As speculation, it certainly seems quite feasible that meteor impacts could eject simple life to other planets/moon in the Solar System.

Read the research article here - Viable Nematodes from Late Pleistocene Permafrost of the Kolyma River Lowland

0

The Threat of Gamma Ray Bursts

Recently I stumbled into a paper estimating the number of supernovae (SN) in our galaxy (Adams et al. 2013). This reminded me of the number of news outlets that latched onto the potential of gamma ray bursts (GRBs) to create a catastrophic extinction event on Earth (here are some examples: Did Deadly Gamma-Ray Burst Cause a Mass Extinction on Earth?, Asteroids are not the only threat to life from space,  and shockingly from Nature -- 'Death Star' found pointing at Earth - that's a serious level of click-bate from such a prestigious journal).

You are welcome to read those articles and I'm not going to go into any deep detail in this post on GRB physics or planetary impacts. However, I did want to do some simple napkin math on what the probability of a direct gamma ray burst from a specific subset of core collapse SN might be on Earth. We're indebted to Dr. Enrico Cappellaro's dedicated research into SN rates within galaxies (see Cappellaro, Evans & Turatto 1999 and associated citations for newer work) for some of the broader rough estimates. There are a number of ways to determine the SN rate, but the most obvious way is to utilize observations across multiple galaxy surveys to measure the rate by look-back time. Other methods include measured pulsar birth rates (some percentage of SNs), the amount of SN remnant isotopes and future neutrino detection.

0

Populating the Galaxy

The Jet Propulsion Laboratory recently hosted the 10th Global Trajectory Optimization Competition or GTOC X with a challenge to see how many star systems could be colonized with the the most uniform spacial distribution. The contest constraints were based on time to populate, a limit of 100,000 predefined stars, a fixed number of colonizing ships and as little propulsive velocity change as possible.

What's most interesting is that the nuances of the challenge require thinking about the motion of the ships as well as the motion of the stars around the center of the galaxy. Each ship has a limit of impulsive maneuvers to successfully rendezvous with a star system which forces some optimization on trajectories of the colonizing ships to keep the propulsive change to a minimum(use this link to the deeper descriptive rules in pdf).

Seems Team 7 from China of the College of Aerospace Science and Engineering, National University of Defense Technology and Key Laboratory of Astronautic Dynamics organizations bested all of the competition by colonizing 3798 systems (well above 2nd place).  You can see the results here.

0